Synthesis and characterization of unsaturated thermotropic polyesters prepared via acyclic diene metathesis polymerization

Acyclic diene metathesis (ADMET) polymerization was employed to prepare thermotropic liquid crystalline polymers (TLCPs) bearing double bonds within flexible spacers. Starting from a diene monomer, 2-tert-butyl-1,4-phenyl bis(4-pentenyloxybenzoate) (5tB), we successfully synthesized unsaturated liquid crystalline oligomers and polymers, P5tBn, via ADMET polymerization. Such unsaturated TLCPs were developed to allow the preparation of tailored, thermally cross-linkable, liquid crystalline thermosets as adhesives and composite matrix resins. High degrees of polymerization were achieved and the resulting polymers showed clear mesomorphic behavior with complete absence of crystallization. Thermal analysis of polymers prepared with varying synthetic conditions showed that TLCPs prepared using different ruthenium metathesis catalysts featured substantially different transition temperatures, implying an influence of the catalyst on the resulting TLCP chain microstructure. In light of these observations, NMR studies (1D ¹H and ¹³C, and 2D COSY and HMQC) were carried out, allowing determination of the selectivity of the catalysts toward trans/cis linkage and coupling/isomerization of units in the polymer chain. Furthermore, by studying the polymerization and isomerization kinetics, we shed light on the cause of isomerization. It was found that the degradation product of Grubbs catalyst is implicated in isomerization catalysis, in agreement with reports on the metathesis of 1-octene. In contrast, second-generation Grubbs catalyst itself (not a degradation product) apparently causes isomerization. For a related catalyst, developed by Nolan, both the catalyst and its degradation product are concluded to be responsible for the isomerization reaction.